1. Technical Field
The present invention relates to data transmission in a control system used to control operation of a machine or a facility.
2. Related Art
Typically, machines and facilities used in production sites are controlled by a control system including a Programmable Logic Controller (hereinafter also referred to as a “PLC”). The control system includes an IO (Input and Output) unit that plays a role to input a signal from an external switch or sensor and output a signal to an external relay or actuator. The IO unit is scheduled to be disposed in various places of the production site.
The IO unit is connected to a processing unit including a processor through a communication line. Sometimes the communication line is constructed as an internal bus. In the case that the internal bus is used, use of a relatively high clock frequency (transmission frequency) can enhance a transmission capability (such as a transmission rate and a transmission band).
For example, Japanese Unexamined Patent Publication No. 2011-192068 discloses a configuration in which data communication is surely conducted at high speed between units constituting the PLC in consideration of an influence of a noise.
Even if the communication line having the high transmission capability is used, possibly a state in which subsequent data cannot be transmitted because preceding data exists, namely, a “transmission waiting” state is generated when many pieces of data are transmitted. On the other hand, a priority corresponding to a content can exist among the pieces of data that are transmitted and received through the communication line.